Avoid up-cast warnings in pngvalid.c
On ARM the alignment requirements of png_modifier are greater than that of
png_store and as a consequence compilation of pngvalid.c results in a warning
about increased alignment requirements because of the bare cast to
(png_modifier*).  The code is safe, because the pointer is known to point to a
stack allocated png_modifier, this change avoids the warning.

Signed-off-by: John Bowler <jbowler@acm.org>